Resins and glasses are used generally for light diffusing plates. For obtaining a light diffusing plate from these materials, an opaque polycarbonate resin, for example, having a light diffusing property in itself is used. Alternatively, a light diffusing property is imparted to a material by processing the surface of the material by acid treatment or sand blasting. Alternatively further, a film having a light diffusing function or an opaque film is provided on a substrate made of a resin or glass.
These materials, however, have a high average linear thermal expansion coefficient. As to glasses, high thermal expansion glasses called a blue plate and a white plate are generally used. When such materials having a high average linear thermal expansion coefficient are used in an environment which undergoes a significant temperature change, there arises the problem that Haze value cannot be maintained at a constant value due to expansion of the material.
These materials, therefore, have sufficient performances as a light diffusing plate when it is used for ordinary measuring instruments but are not suitable for a light diffusing plate of a measuring instrument used for measuring diffusing light accurately in an environment which undergoes a significant temperature change.
Even in an environment in which the temperature is constant, in case laser beam, for example, is used as the light source of diffusing light, the temperature of a light diffusing plate is elevated resulting in instability in Haze value. There is also a problem of damage to a light diffusing plate due to thermal shock caused by laser irradiation.
As a substrate for a magnetic information recording medium, several types of glass-ceramics having a small average linear thermal expansion coefficient have been proposed by, for example, Japanese Patent Application Laid-open Publication Nos. 2002-284544, 2002-308647 and Hei 11-314939. In these glass-ceramics, however, a crystal grain diameter of precipitating crystals is a small one of 0.1 μm or below and therefore the light diffusing property of these glass-ceramics is generally poor.
Japanese Patent Application Laid-open Publication No. 2004-131372 discloses lithium aluminosilicate glass-ceramics. The glass-ceramics have a large average linear thermal expansion coefficient within a range from 15×10−7/° C. to 30×10−7/° C. within a temperature range of 20-700° C. and therefore have an instable Haze value in an environment which undergoes a temperature change.
Japanese Patent Application Laid-open Publication No. Hei 7-172862 discloses glass-ceramics which have a low average linear thermal expansion coefficient within a range from −1×10−6K−1 to +2×10−6K−1 within a temperature range of −50-700° C. The glass-ceramics, however, contain Na2O or K2O and, when a film is formed on a substrate made of the glass-ceramics, the movable alkali components solve out of the substrate with lapse of time and contaminate the film.
It is, therefore, an object of the invention to provide glass-ceramics which have a low thermal expansion property with little expansion and contraction against change in the temperature and therefore have a stable haze value and an excellent light diffusing property and exhibit an opaque or white color.
It is another object of the invention to provide glass-ceramics elements using the glass-ceramics.
It is another object of the invention to provide a method for manufacturing the glass-ceramics.
It is still another object of the invention to reduce solving out of alkali components with lapse of time.